Import MenuItem from its new location

Since I5b879c4fa, MenuItem moved so import it to fix compilation error.

Change-Id: I231ce535c32c649ff8febf911d1b280db33b0186

src/main/java/com/googlesource/gerrit/plugins/cookbook/HelloTopMenu.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/cookbook/HelloTopMenu.java b/src/main/java/com/googlesource/gerrit/plugins/cookbook/HelloTopMenu.java
index 96e22d8..8734b7c 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/cookbook/HelloTopMenu.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/cookbook/HelloTopMenu.java
@@ -16,6 +16,7 @@
 
 import com.google.common.collect.Lists;
 import com.google.gerrit.extensions.annotations.PluginName;
+import com.google.gerrit.extensions.client.MenuItem;
 import com.google.gerrit.extensions.webui.TopMenu;
 import com.google.inject.Inject;